The Electroencephalogram (EEG) is a non-invasive medical test that measures the electrical activity produced by the brain. Electrodes placed on the scalp detect voltage fluctuations resulting from the summed activity of millions of cortical neurons. The primary purpose of the EEG is to diagnose neurological conditions, particularly those involving electrical dysfunction like seizures and epilepsy. Clinicians inspect the recorded waveforms to gain insight into the functional state of the brain.
Deciphering the Basic Components of the EEG Trace
The EEG trace is a visual representation of brain activity over time, composed of three fundamental elements: frequency, amplitude, and localization. Frequency refers to the speed of the brain waves, measured in Hertz (Hz), or cycles per second. A high-frequency rhythm shows many waves passing a point quickly, while a low-frequency rhythm indicates a slow speed.
Amplitude is the height or size of the waves, reflecting the voltage or strength of the electrical signal, measured in microvolts. Larger, higher-amplitude waves suggest a greater number of neurons are firing in a synchronized manner.
Activity is tracked across numerous channels corresponding to electrode placement on the scalp, typically following the International 10-20 system. This placement allows activity to be localized to specific regions of the brain, such as the frontal, temporal, or occipital lobes.
Establishing the Normal Baseline Rhythms
Interpreting an EEG begins with identifying the normal background rhythms, classified into four main frequency bands based on cycles per second.
Beta Activity
The fastest rhythm is Beta activity, defined as frequencies greater than 13 Hz. It is usually seen when a person is awake, alert, and actively thinking or concentrating. Beta activity is typically most prominent over the frontal and central regions of the scalp and generally appears with low amplitude.
Alpha Activity
Alpha activity operates in the mid-range of 8 to 13 Hz and is the dominant rhythm when a healthy adult is awake but relaxed, particularly with their eyes closed. This rhythm is found over the posterior regions of the head and should disappear or “block” when the eyes open or the patient is mentally aroused. A healthy background rhythm should also be symmetric, meaning the frequency and amplitude are largely equal across both hemispheres.
Theta Activity
Slower rhythms are normally associated with states of reduced consciousness, such as sleep. Theta activity falls in the 4 to 7 Hz range and is common during drowsiness and light sleep stages in adults. This frequency range is considered a normal finding in young children up to about 13 years of age, often dominating their awake background activity.
Delta Activity
The slowest rhythm is Delta activity, defined as frequencies of less than 4 Hz. It is the hallmark of deep, non-REM sleep in adults and the expected dominant activity in infants and toddlers. The significance of any rhythm is assessed not just by its frequency, but by the patient’s age and state of consciousness at the time of the recording.
Recognizing Common Patterns of Deviation
Pathological EEG findings are recognized when the waveform deviates significantly from the expected normal patterns for the patient’s age and state of alertness.
Epileptiform Activity
A primary sign of underlying seizure potential is the presence of epileptiform activity, which appears as brief, high-amplitude transients. These discharges are characterized as “spikes” (lasting less than 70 milliseconds) or “sharp waves” (duration between 70 and 200 milliseconds). These pointed waves often interrupt the normal background rhythm and are frequently followed by a prominent slow wave, forming a “spike-and-slow-wave” complex.
Discharges occurring in a restricted area are focal, suggesting localized irritable brain tissue. Generalized epileptiform activity appears simultaneously across both hemispheres, suggesting widespread electrical dysfunction.
Generalized Slowing
Generalized slowing occurs when the normal Alpha and Beta rhythms of an awake adult are replaced by an excessive amount of slower Theta or Delta activity. This replacement indicates a diffuse dysfunction of the cerebral cortex, often seen in conditions like encephalopathy, which can be caused by toxic-metabolic derangements or systemic illness. The severity of the underlying condition often correlates with the degree of slowing; continuous Delta activity is associated with more severe impairment.
Asymmetry
A lack of symmetry between the two hemispheres is a sign of deviation. A significant difference in the amplitude or frequency of rhythms between the left and right sides suggests a focal problem. For example, if one hemisphere displays normal background activity but the other shows greatly reduced amplitude or excessive slow wave activity, it can indicate a structural lesion such as a stroke, tumor, or hematoma that is interfering with the electrical signals.